1. Field of the Invention
The preferred embodiments of present invention relate to, among other things, a reverse mounted leadless semiconductor device and, more particularly, to a semiconductor device and a method for manufacturing the same that can be employed to reduce deficiencies caused when mounting the semiconductor device.
2. Description of the Related Art
The following description sets forth the inventors' knowledge of related art and problems therein and should not be construed as an admission of knowledge in the prior art. As semiconductor device capacities have been increased year-by-year, the number of required lead terminals, which serve as various signal lines, has tended to increase. This tendency has resulted in greater use of semiconductor devices such as QFP (Quad Flat Package) semiconductor devices having lead terminals extending from their four sides and QFN (Quad Flat Non-leaded Package) semiconductor devices. For example, one practical example of a method for manufacturing a QFP semiconductor device is disclosed in Japanese Unexamined Patent Publication No. Hei-8-181160.
Illustrative existing methods for manufacturing a semiconductor device are now described below with reference to FIGS. 12–15. In that regard, FIG. 12 is a plan view illustrating a lead frame. FIG. 13 is a perspective view illustrating a mold. And, FIG. 14 is a plan view illustrating a lead frame after having been encapsulated with plastic.
First, as shown in FIG. 12, a semiconductor element is mounted, via silver paste serving as a bonding agent, on a stage 2 of a lead frame 1. Although not illustrated, the semiconductor element has a plurality of electrode portions on its surface and is mounted on the stage to be fixedly attached thereto. Thereafter, the electrode portions are electrically connected to lead terminals 3 using wire bonding.
As shown in FIG. 13, after the semiconductor element has been mounted as described above, the lead frame 1 is placed in between an upper mold 7 and a lower mold 8. Thereafter, closing the molds causes a cavity to be defined which serves as an injection volume.
Then, a melted plastic is injected at a predetermined pressure therein through a pot 10′ of the upper mold 7. The plastic flows into the cavities of the upper mold 7 and the lower mold 8, filling in a cavity 9 via a runner 11, thereby encapsulating the semiconductor element. Although air exists inside the cavity 9 before the plastic is injected, the plastic pushes the air out through an air vent at the stage of the plastic penetrates the cavity. Then, the air flows outside through a hole 5 formed in the lead frame 1. The air vent is formed in the molds 7 and 8 and has a sufficient extent of clearance not to allow the plastic to pass therethrough.
After the plastic filled has cooled down and solidified, the molds are opened to take out the lead frame 1. The lead frame at this point in time is shown in FIG. 14. In this figure, to clarify the flow passage of the plastic, portions where the pot and runner were present at the time of plastic encapsulation are shown with dashed lines. As can be seen clearly from FIG. 14, the plastic flows into the molds from a pot portion 10 that is located at the center of four encapsulation regions through a gate portion 4. This allows the semiconductor element to be mounted on the stage and part of the lead terminals 3 located around the periphery of the semiconductor element to be covered with the plastic, thereby forming a package 12. Thereafter, joint portions of the lead terminals 3 are cut off, and the separated lead terminals 3 are bent as necessary to thereby complete a QFP semiconductor device.
Next, FIGS. 15(A) and 15(B) illustrate a QFP semiconductor device that has been formed by the same method as that for manufacturing the aforementioned QFN semiconductor device.
FIG. 15(A) is a cross-sectional view illustrating a semiconductor device including a lead 15 formed portion. As illustrated, this background semiconductor device is configured such that a semiconductor element 16 is fixedly attached to an island 14 formed of a Cu frame via an electrically conductive paste 17 such as silver (hereinafter referred to as Ag) paste. An electrode pad (not shown) of the semiconductor element 16 is electrically connected to the lead 15 via a thin metal wire 18. In addition, an insulating resin 19, which integrally covers the semiconductor element 16 and other components, is formed on the island 14 and the lead 15 made of a Cu frame. Then, the reverse surface of the island 14 and the lead 15 is plated for prevention of oxidation and solder wettability. With this structure, for example, the lead 15 is mounted to a mounting substrate (not shown) via solder. At this time, the reverse surface of the semiconductor device is formed to be generally flush therewith, ensuring that the semiconductor device is mounted on the mounting substrate with stability.
Now, FIG. 15(B) is a cross-sectional view illustrating a semiconductor device including a lifting lead 13 formed portion. As illustrated, on the upper surface of the lifting lead 13 exposed on the side surface of the insulating resin 19, plastic burrs 19A are produced continuously on the side surface of the insulating resin 19. These burrs are the plastic that has flowed into the air vent portion provided in the molds and hardened, for example, with a thickness of approximately 30 μm.
As described above, the mounting surface of the semiconductor device is formed to have generally the same plane as shown in FIG. 15(A) in the background QFN semiconductor device. For this reason, when the semiconductor device is mounted onto the mounting substrate, mounting deficiencies are caused by dust particles such as plastic particles entering in between the substrate and the semiconductor device.
Furthermore, as described above, in the method for manufacturing a background semiconductor device, the air present in the cavity 9 is driven towards the end portion of the cavity 9, from which the air passes outwardly through the air vent provided in the molds, as shown in FIG. 13. However, when the air is pushed out via the air vent, the plastic turns into burrs between the lead frame 1 and the upper mold 7 or between the lead frame 1 and the lower mold 8. To cut the package 12 out of the lead frame 1, the peripheral portion of the package 12 is cut while being fixed. However, as shown in FIG. 15(B), when plastic burrs 19A have occurred on this fixed region, especially on the surface of the lifting lead 13, it can be impossible to reliably secure the leads 3. As a result, on the cutting surface of the plastic formed between the leads 3, microcracks can be produced. In subsequent steps, these cracks will turn to be plastic particles, etc., which would induce mounting deficiencies in the mounting step.
Furthermore, in the method for manufacturing a background semiconductor device, the air present in the cavity 9 is driven towards the end portion of the cavity 9, from which the air passes the cavity 9 outwardly through the air vent provided in the mold 7. However, when the air is pushed out via the air vent, the plastic turns into burrs between the lead frame 1 and the upper mold 7 or between the lead frame 1 and the lower mold 8. Because the plastic burrs are as thin as approximately 30 μm, the plastic burrs are integrated with the package and may remain in the mold when the package is removed from the mold 6. The plastic burrs remaining in the mold may block the passage of air present in the cavity 9 at the time of the subsequent plastic molding. As a result, because the air does not flow outside and thus remains compressed in the cavity 9, such a problem can arise wherein voids and/or unfilled volumes occur in the package.
There is a need in the art for improved systems and methods that overcome the above and/or other problems.